THE THEORY OF MOSAIC VISION, 123 
Each visual pyramid, isolated from its fellows by its coat 
of pigment, may be supposed, in fact, to play the part of a 
very narrow straight tube, with blackened walls, one end 
of which is turned towards the external world, while the 
other incloses the extremity of one of the nerve fibres. The 
only light which can reach the latter, under these circum- 
stances, is such as proceeds from points which lie in the 
Fig. 29.—Diagram showing the course of rays of light from three 
points z, ¥, z, through the nine visual rods (supposed to be empty 
tubes) A—I of a compound eye ; a—i, the nerve fibres connected 
with the visual rods. 
direction of a straight line represented by the produced 
axis of the tubes. 
Suppose A—I to be nine such tubes, a—i the corre- 
sponding nerve fibres, and a y z three points from which 
light proceeds. Then it will be obvious that the only light 
